Studies are being carried out on various aspects of toxigenic lipid peroxidation. It is known that initially localized lipid peroxidation leads to pathological changes at distant sites. We have shown that peroxidative decomposition of rat liver microsomal lipids yields a dialyzable toxic product (or products), which lyses red cells from vitamin E deficient rats. Work is underway to concentrate, purify, and identify the active dialyzable compounds. Toxic products can also be extracted with organic solvents from sedimented rat liver microsomes which have undergone lipid peroxidation. These lipid soluble toxic products are stable in organic solvents, but they are unstable (t1/2 equals approx. 15 min) in aqueous solvents. Studies are underway to concentrate, purify, and identify the toxic products.